1. Field of the Invention
The present invention relates generally to wireless communications and, more particularly, to a Channel Quality Indicator (CQI) transmission power control method for a wireless communication system supporting multi-cell High Speed Downlink Packet Data Access (HSDPA) in which multiple cells transmit packet data to a user equipment.
2. Description of the Prior Art
Universal Mobile Telecommunications System (UMTS), which is one of the third generation (3G) mobile telecommunication technologies based on the Wideband Code Division Multiple Access (WCDMA), has evolved from Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS) for providing mobile and computer users with packet-based multimedia services including text messaging and voice and video communication services in a consistent service environment.
The UMTS system supports HSDPA service to enhance the packet data transfer speed and capacity in downlink.
In order to secure high speed data transfer rate, HSDPA uses Adaptive Modulation and Coding (AMC) and Hybrid Automatic Repeat Request (HARQ). With the AMC, the Quadrature Phase-Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (16QAM), and 64QAM modulation schemes are selectively used. The AMC technique selects the modulation scheme and coding format and coding rate on the basis of the channel condition and channel type between the Base station and the user equipment (UE) so as to improve the entire cell throughput.
HARQ is implemented with fast retransmission and soft combining techniques. When an erroneous packet is detected, the retransmission is triggered between the base station and UE, and the recipient part combines the retransmitted packet with the previously received packets, thereby reducing the number of retransmission requests to improve overall efficiency. In order to support the HSDPA service, the base station and UE exchange control information such as Orthogonal Variable Spreading Factor (OVSF) codes and number of OVSF codes, Transport Block Sixe (TBS), Modulation and Coding Scheme (MCS), channel index information for HARQ, CQI for indicating the channel condition, and HARQ ACK/NACK.
FIG. 1 is a sequence diagram illustrating operations of a cell and a UE in HSDPA system.
Referring to FIG. 1, first the UE 102 transmits a Channel Quality Indicator (hereinafter called CQI) to a cell 101. Since the UE 102 does not know when the data are transmitted in downlink, it transmits the CQI information periodically (103). When there are data to be sent, the Node B 101 performs scheduling based on the CQI. In the scheduling process, the Node B determines a number of code channels available for allocation and an MCS level. Such information is transmitted to the UE 102 through a High Speed Shared Control Channel (HS-SCCH) (105). The HS-SCCH is received by the UE 102 in a TTI, and the UE 102 receives data by demodulating the HS-PDSCH 106 with reference to the HS-SCCH. In order to make a status report for Hybrid Automatic Repeat Request (HARQ), the UE 102 performs Cyclic Redundancy Check (CRC) to determine Acknowledgement/Non-Acknowledgement (ACK/NACK) (103). If the data are received in error, the UE 102 transmits a NACK to Node B 101 to request retransmission of the data; and otherwise, an ACK to Node B 101 (107). The status reports of ACK/NACK and CQI are transmitted through a High Speed Dedicated Physical Control Channel (HS-DPCCH).
FIG. 2 is a timing diagram illustrating transmissions of the physical channels of an HSDPA system. As shown in FIG. 2, the CQIs 205, 206, and 207 are periodically transmitted via the HS-DPCCH. Node B transmits two slots of the HS-SCCH before it begins transmitting the HS-PDSCH in order for the UE to check the information on the demodulation of the HS-PDSCH. The ACK/NACK information 204 is transmitted 7.5 slots 203 after the transmission of the HS-PDSCH 202 in consideration of the demodulation and decoding of the data carried by the HS-PDSCH.
The Dual-Cell HSDPA operation is described hereinafter with reference to FIG. 3. Unlike the conventional HSDPA in which the UE measures received signals strengths of the cells and connects to the most appropriate cell based on the measurements, the Dual-Cell HSDPA is characterized in that the UE 308 connects to two different cells 301 and 302 defined by two different carriers 303 and 304 of a Node B. The UE 308 receives the HSDPA signals from the second cell 302 in the first carrier f1 304 and from the first cell 301 in the second carrier f2 303, simultaneously. In the WCDMA system, the transmission bandwidth of a cell is 5 MHz such that the UE must have a reception capability of 10 MHz for supporting Dual-Cell HSDPA. Since the HSDPA signals are received from two cells, the maximum transmission rate increases twice. In case of uplink, however, the Dual-Cell transmission function is not supported, whereby the uplink channel is transmitted to only one cell. Even in downlink transmission, common and dedicated channels that are not related to the HSDPA are received from a single cell. Typically, the cell which is in charge of controlling the uplink channel and common and dedicated downlink channels is called an “anchor cell” and the other cell is called a “supplementary cell”.
Although the 3GPP standard specifies the Dual-Cell HSDPA service with two cells (including one anchor cell and one supplementary cell) as of year 2008, a multi-cell HSDPA can be implemented with the involvement of more than two cells (including one anchor cell and two or more supplementary cells).
In order for the Dual-Cell HSDPA service to support the HARQ and AMC, the ACK/NACK and CQI should be transmitted to the respective cells, whereby the uplink channel permitted to the anchor cell must be configured to carry the ACK/NACKs and CQIs destined for the anchor and supplementary cells. In this case, the CQIs for the anchor and supplementary cells (each is 5-bit long) are concatenated into 10-bit control information and encoded with a (20, 10) block code. The block coded control information is transmitted through the HS-DPCCH specified for the CQI and ACK/NACK transmission. In case that the Dual-Cell HSDPA operation is not configured, the 5-bit CQI for the anchor cell is encoded with a (20, 5) block code and then transmitted through the HS-DPCCH specified for the CQI and ACK/NACK transmission. The Dual-Cell HSDPA operation is signaled by the base station to the UE.
However, detailed uplink control information transmission method for the multi-cell HSDPA has not been discussed yet until now.